Synthesis And Opto-electronic Properties Of Transition-metal-doped Semiconductor Nanocrystals

Semiconductor nanocrystals (NCs) have been widely studied for their fundamental properties and applications, mostly as tunable emitters for biomedical labeling, light emitting diodes, lasers, and sensors. Despite their apparent advantages versus organic dyes, the intrinsic toxicity of cadmium has casted a doubtful future for this promising field. Zinc chalcogenide doped with transition metal ions may generate a cadmium-free nanocrystal emitter and the large ensemble Stokes shift can avoid the reabsorption or the self-absorption process and make the system ideal for different optical applications. We synthesized stable, small, efficient transition metal ions doped Zinc chalcogenide quantum dots by femtosecond laser ablation as well as nucleation-doping strategy. We study on their growth kinetics and luminescent property. The original works are organized as follows:1. We synthesized MnS/ZnS core/shell quantum dots (QDs) or called ZnS:Mn QDs via hot solution phase chemistry using nucleation-doping strategy. Efficient PL of Mn ions in the QDs with quantum yield (QY) of about 40-50% is demonstrated in the resulting QDs. The experimental results indicated that highly efficient luminescent ZnS:Mn NCs could be obtained by controlling the diffusion of Mn ions into the ZnS shell via annealing the core/shell NCs and using small sized MnS cores In addition, we also prepared ZnSe:Cu QDs with the PL QY=10% using growth-doping strategy.2. We studied the mechanism of photoluminescence (PL) from MnS/ZnS core/shell QDs synthesized using nucleation-doping strategy. The experimental results indicate that the mechanism for improving the PL QY in MnS/ZnS QDs with thick ZnS shell can be understood in terms of significantly enhanced energy transfer from the ZnS shell to Mn ions and slightly decreased nonradiative relaxation rate from Mn ions to surface states/traps of the ZnS shell by the surface passivation of the QDs with a thick ZnS shell.3. We studied the diffusion of Mn ions in MnS/ZnS nanocrystals by steady-state and time-resolved photoluminescence spectroscopy. Based on the evolution of the Mn dopant photoluminescence (PL) and its lifetime, we confirmed that the Mn ions can diffuse to the surface of the ZnS shell from the MnS core under long annealing at the temperature above 220°C, resulting in a decrease in PL QY and lifetime. It is found that the diffusion process at the initial growth of the ZnS shell is a key factor for the interface layer between the MnS core and ZnS shell, thus determine the quality of the obtained nanocrystals. We thus modified the synthesis procedure as follows to obtain highly efficient luminescent MnS/ZnS QDs: growing a thin ZnS shell on a small sized MnS core at low temperature, annealing the resulting NCs for effectively diffusing Mn ions into the ZnS shell at high temperature to form a ZnS:Mn diffusion layer, and overcoating a thicker ZnS shell as a passivating layer.4. The ligand exchange of the MnS/ZnS QDs was carried out to transfer the QDs into a water solution by using mercaptopropionic acid for biomedical applications. Recognition of a biotin pattern by QDs conjugated with avidine was carried out to illustrate the suitability of these efficient, stable, small, and water soluble QDs as biomedical labeling reagents.5. We prepared Cu-doped ZnS (ZnS:Cu) QDs in de-ionized water by femtosecond laser ablation of a bulk ZnS:Cu target. The obtained QDs exhibit good colloidal stability and water solubility, showing narrow and symmetric Cu-related emission. The mean size of the quantum dots varies from 2.1 to 4.0 nm by changing the laser fluence, resulting in a redshift of the emission peak from 375 to 400 nm. The experimental results demonstrate that femtosecond laser ablation is an effective method for preparing doped QDs.